Process for manufacturing a microbolometer containing vanadium oxide-based sensitive material

ABSTRACT

A process for manufacturing at least one microbolometer comprising a sensitive material based on vanadium oxide containing nitrogen as additional chemical element, includes steps of determining a non-zero effective amount of the additional chemical element starting from which the sensitive material, having undergone a step of exposure to a temperature Tr for a duration Δtr, has an electrical resistivity ρa|r at ambient temperature greater than or equal to 50% of the native value ρa of said sensitive material at ambient temperature; producing the sensitive material in a thin layer having an amount of the additional chemical element greater than or equal to the effective amount determined beforehand, the sensitive material being amorphous and having an electrical resistivity of between 1 and 30 Ω·cm; and exposing the sensitive material to a temperature Tr for a duration Δtr.

TECHNICAL FIELD

The field of the invention is that of devices for detectingelectromagnetic radiation, for example infrared or terahertzelectromagnetic radiation, comprising at least one resistive thermaldetector comprising a sensitive material based on vanadium oxide. Theinvention is applicable notably to the field of infrared imaging andthermography.

This invention is the result of a contract concluded with the FrenchMinistry of Defense, which possesses certain rights thereto.

PRIOR ART

An electromagnetic radiation detection device may comprise an array ofresistive thermal detectors, also called microbolometers, eachmicrobolometer comprising an absorbent portion capable of absorbing theelectromagnetic radiation to be detected.

In order to thermally insulate the sensitive material of themicrobolometers, the absorbent portions are usually in the form ofmembranes suspended above the substrate by anchoring pillars, and arethermally insulated therefrom by holding and thermal insulation arms.These anchoring pillars and thermal insulation arms also have anelectrical function by electrically connecting the suspended membranesto a readout circuit that is generally arranged in the substrate.

The absorbent membrane comprises a sensitive material whose electricalresistivity p varies as a function of the temperature of the material.The sensitive material is characterized by the value pa of theelectrical resistivity at ambient temperature and by its coefficient a(or TCR), which is defined by the relationship α=1/ρ×dρ/dT. Thesensitive material may be a semiconductor material usually chosen fromamong amorphous silicon and a vanadium oxide VO_(x).

The choice of the sensitive material depends notably on itscompatibility with the conventional deposition and etching steps usuallyused in microelectronics, and notably in silicon technology. However, itappears that a sensitive material based on vanadium oxide is likely tohave its electrical properties degraded following the microbolometermanufacturing process.

DISCLOSURE OF THE INVENTION

The aim of the invention is to at least partly remedy the drawbacks ofthe prior art, and more particularly to propose a process formanufacturing at least one microbolometer comprising a sensitivematerial based on vanadium oxide whose electrical properties arepreserved, and more precisely whose risks of i/f noise degradation ofthe sensitive material, following the manufacturing process, are limitedor even eliminated.

To this end, the subject of the invention is a process for manufacturingat least one microbolometer comprising a sensitive material for at leastlimiting noise degradation of said sensitive material,

-   -   said sensitive material being formed of a first compound based        on vanadium oxide and at least nitrogen as additional chemical        element,    -   the process comprising the following steps:        -   a step of producing the sensitive material in a thin layer;        -   a step of exposing the sensitive material to a temperature            T_(r) greater than the ambient temperature, for a duration            Δt_(r), this thermal exposure step being performed after the            step of producing the sensitive material,            -   the temperature T_(r) and the duration Δt_(r) being such                that said first compound, being amorphous and having a                native electrical resistivity value at ambient                temperature of between 1 Ω·cm and 30 ∩·cm, having                undergone a step of exposure to the temperature T_(r)                for the duration Δtr, has an electrical resistivity at                ambient temperature less than 50% of its native value;    -   the process furthermore comprising the following steps:        -   i. determining a non-zero what is called effective amount of            the additional chemical element added to said first            compound, thus forming a modified compound, starting from            which the modified compound, having undergone a step of            exposure to the temperature T_(r). for the duration Δt_(r),            has an electrical resistivity ρ_(a|r) at ambient temperature            greater than or equal to 50% of the native value ρ_(a) of            said sensitive material at ambient temperature;    -   ii. in said step of producing the sensitive material in a thin        layer, the latter is formed of said modified compound having an        amount of the additional chemical element greater than or equal        to the effective amount determined beforehand, the sensitive        material being amorphous, having a native electrical resistivity        value ρ_(a) at ambient temperature of between 1 Ω·cm and 30        Ω·cm, and a homogeneous chemical composition;    -   iii. such that, following said step of exposing the sensitive        material to the temperature T_(r) for the duration Δt_(r), said        sensitive material (15) then has a noise whose degradation has        been at least limited.

Certain preferred but non-limiting aspects of this manufacturing processare as follows.

The manufacturing process may comprise a preliminary step of determiningthe native value ρ_(a) of the electrical resistivity at ambienttemperature of the sensitive material containing the non-zero amountunder consideration of the additional chemical element. It may alsocomprise a preliminary step of determining the native value of theelectrical resistivity at ambient temperature of the first compound.

The step of exposing the sensitive material may comprise a step ofdepositing a protective layer covering the sensitive material.

The step of exposing the sensitive material may comprise a step ofdepositing an encapsulation layer transparent to the electromagneticradiation to be detected and intended to define a cavity in which themicrobolometer is located.

The temperature T_(r) may be greater than or equal to 280° C., or evenequal to 310° C. to within 5° C.

The duration Δt_(r) may be greater than or equal to 90 min.

The sensitive material may be produced at a temperature less than thetemperature T_(r), for example at ambient temperature.

The invention also relates to a microbolometer comprising a sensitivematerial made of a first compound based on vanadium oxide and at leastnitrogen as additional chemical element. The sensitive material:

-   -   is amorphous,    -   has an electrical resistivity at ambient temperature of between        1 Ω·cm and 30 Ω·cm,    -   a homogeneous chemical composition, and    -   an amount of nitrogen, defined as the ratio of the number of        nitrogen atoms to that of vanadium, at least equal to 0.070.

The amount of oxygen, defined as the ratio of the number of oxygen atomsto that of vanadium, is preferably between 1.42 and 1.94, to within plusor minus 0.05. Moreover, the electrical resistivity at ambienttemperature of the sensitive material may be between 2 Ω·cm and 30 Ω·cm,and its amount of oxygen may then be between 1.56 and 1.94, to within0.05. The amount of nitrogen may then be at least equal to 0.073.

The sensitive material may be covered by a protective layer of siliconnitride.

The invention also relates to a device for detecting electromagneticradiation, comprising an array of microbolometers according to any oneof the above features.

The microbolometers may be arranged in at least one hermetic cavitydelimited by an encapsulation structure transparent to theelectromagnetic radiation to be detected, the encapsulation structurecomprising at least one layer made of amorphous silicon.

The detection device may comprise a getter material located in thehermetic cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, aims, advantages and features of the invention willbecome more dearly apparent on reading the following detaileddescription of preferred embodiments thereof, this description beinggiven by way of non-limiting example and with reference to the appendeddrawings, in which:

FIGS. 1A and 1B are schematic and partial views, respectively inperspective and in section along the plane A-A, of a microbolometeraccording to one embodiment, comprising a sensitive material based onvanadium oxide;

FIGS. 2A to 2C illustrate, respectively: o an example of the evolutionof the electrical resistivity at ambient temperature of a base compoundproduced from vanadium oxide, not containing nitrogen, as a function ofa thermal exposure temperature T_(r);

-   -   an example of values of the TCR coefficient for VO_(x) base        compounds as a function of their electrical resistivity, without        thermal exposure, and after thermal exposure to 310° C. for 90        min;    -   an example of values of a parameter representative of the 1/f        noise for VO_(x) base compounds as a function of their        electrical resistivity, without thermal exposure, and after        thermal exposure to 310° C. for 90 min;

FIGS. 3A and 3B are examples of Raman spectra for VO_(x) base compoundswithout thermal exposure, and after thermal exposure for 90 min atvarious temperatures;

FIG. 4 is a graph illustrating the evolution of the electricalresistivity par at ambient temperature of the sensitive material basedon vanadium oxide, after exposure of said material to a temperatureT_(r) for a duration Δt_(r), in the case where the sensitive materialcomprises, or does not comprise, a sufficient amount of nitrogen.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

In the figures and in the remainder of the description, the samereferences represent identical or similar elements. In addition, thevarious elements are not shown to scale so as to improve the clarity ofthe figures. Moreover, the various embodiments and variants are notmutually exclusive and may be combined with one another. Unlessindicated otherwise, the terms “substantially”, “approximately”, “of theorder of” mean to within 10%, and preferably to within 5%, and, withregard to temperatures, to within 10° C. and preferably to within 5° C.Moreover, the expression “comprising a” should be understood to mean“comprising at least one”, unless indicated otherwise.

The indications relating to the chemical composition of a compound areexpressed by its empirical chemical formula, conventionally expressedwith respect to one (1) vanadium atom. Thus, for a compound VO_(x)B_(y),mentioned here purely by way of illustration, the value x of the amountof oxygen is the number of oxygen atoms per 1 vanadium atom, and thevalue y of the amount of boron is the number of boron atoms per 1vanadium atom. The value of the amount of the chemical element is givento within 10%. Moreover, the atomic percentage of each chemical elementin the compound VO_(x)B_(Y) is 1/(1+x+y) for vanadium, x/(1+x+y) foroxygen, and y/(1+x+y) for boron.

The invention relates notably to a process for manufacturing at leastone resistive thermal detector, also called a microbolometer, comprisinga sensitive material based on vanadium oxide VON. The microbolometer maybe designed to detect infrared or terahertz radiation. The sensitivematerial comprises a sufficient non-zero amount of at least oneadditional chemical element, here nitrogen and possibly, in addition tonitrogen, boron B and/or carbon C. In addition, the manufacturingprocess implements at least one step in which the sensitive material isexposed to a temperature greater than the ambient temperature and lessthan or equal to the temperature T_(r), for a duration less than orequal to Δt_(r) for example to 300° C. for 10 min, 30 min, or even 90min or more. The thermal exposure temperature T_(r) is therefore higherthan the ambient temperature.

This thermal exposure step may correspond to the deposition, atapproximately 300° C., of a protective layer, for example made of asilicon nitride SiN or a silicon oxide SiO, covering the sensitivematerial in order to protect it from any subsequent contamination bychemical elements that are possibly present. It may also correspond toproducing an encapsulation layer of amorphous silicon, said layer beingintended to define a hermetic cavity in which the microbolometer islocated. It may also involve activating, at approximately 300° C., agetter material located in the hermetic cavity, this getter materialbeing intended to react with residual gas possibly present in the cavityin order to keep same at a sufficient vacuum level.

These examples are given by way of illustration. The step of thermalexposure to the temperature T_(r) for the duration Δt_(r) may generallybe implemented within the framework of technological steps formanufacturing the microbolometer that are performed after the sensitivematerial has been produced, or even within the framework oftechnological steps for manufacturing the detection device after themicrobolometer or microbolometers have been produced, in particular inorder to integrate additional functionalities in the detection chip.

FIGS. 1A and 1B are schematic and partial views, respectively inperspective and in section along the plane A-A, of a microbolometer 10of a device 1 for detecting electromagnetic radiation, themicrobolometer 10 comprising a sensitive material 15 based on vanadiumoxide VO_(x).

The microbolometer 10 comprises an absorbent membrane 11 containingsensitive material 1 ₅ based on vanadium oxide VO_(x), suspended above asubstrate 2 by anchoring pillars 12 and thermal insulation arms 13, aswell as an electronic control and readout circuit (not shown) located inthe substrate 2. The microbolometer 10 is designed here to absorbinfrared radiation contained within the long wavelength infrared (calledLWIR) band, ranging from approximately 8 μm to 14 μm.

Here and for the remainder of the description, a directthree-dimensional orthogonal reference system (X, Y, Z) is defined,where the plane XY is substantially parallel to the plane of a substrate2, the Z-axis being oriented in a direction substantially orthogonal tothe plane of the substrate 2. Moreover, the terms “lower” and “upper”are understood to relate to an increasing position when moving away fromthe substrate 2 in the direction +Z.

The microbolometer 10 comprises a substrate 2 based in this example onsilicon, comprising an electronic circuit (not shown) allowing themicrobolometer to be controlled and read. The electronic circuitcomprises portions of conductive lines, for example made of metal,separated from one another by a dielectric material, for example asilicon-based mineral material such as a silicon oxide SiO, a siliconnitride SiN, or alloys thereof. To this end, it may comprise activeelectronic elements, for example diodes, transistors, capacitors,resistors, etc., connected by electrical interconnections to themicrobolometer 10, on the one hand, and to an interconnection pad (notshown), on the other hand, the latter being intended to electricallyconnect the detection device 1 to an external electronic device.

The upper face of the substrate 2 may be covered with a protective layer(not shown), notably when the absorbent membrane is produced on amineral sacrificial layer, which is then eliminated through a chemicalattack with an acid medium. It may cover or be covered by a reflectivelayer 14 arranged under the absorbent membrane 11. When it covers thereflective layer 14, it is made of a material that is at least partiallytransparent to the electromagnetic radiation to be detected. Theprotective layer has an etch stop function, and is designed to protectthe substrate and the inter-metal dielectric layers, when they are madeof a mineral material, against a chemical attack, for example a chemicalattack with an HF (hydrofluoric acid) acid medium implementedsubsequently in order to etch the mineral sacrificial layer used in theproduction of the absorbent membrane. This protective layer thus forms ahermetic and chemically inert layer. It is electrically insulating so asto avoid any short circuit between the metal line portions. It may thusbe made of alumina Al₂O₃, or even of aluminium nitride or fluoride. Itmay have a thickness of between a few ten and a few hundred nanometers,for example of between 10 nm and 500 nm, preferably of between 10 nm and30 nm.

The microbolometer 10 comprises an absorbent membrane 11 incorporating asensitive material 15 based on vanadium oxide VO_(x), suspended abovethe substrate 2 by anchoring pillars 12 and thermal insulation arms 13.The anchoring pillars 12 are electrically conductive, and locally passthrough the protective layer in order to create electrical contact withthe electronic circuit. The absorbent membrane 11 is spaced from thesubstrate 2, and in particular from the reflective layer 14, by anon-zero distance. This distance is preferably adjusted so as to form aquarter-wave cavity optimizing the absorption of the electromagneticradiation to be detected by the suspended membrane

As illustrated in FIG. 1B, the absorbent membrane 11 may comprise alower support layer 20 made of an electrically insulating material onwhich there rest two electrodes 21.1, 21.2 that are distinct from oneanother and made for example of TiN, which exhibits good absorption ofinfrared radiation. A thin layer of sensitive material 15 rests on thesupport layer 20 and comes into contact with each of the two electrodes21.1, 21.2. The sensitive material 15 is in this case covered with aprotective layer 22, made for example of a silicon nitride SiN or asilicon oxide SiO, which makes it possible to avoid any subsequentcontamination of the sensitive material 15. This example is given purelyby way of illustration, and other arrangements of the electrodes and ofthe sensitive material are possible.

Moreover, the microbolometer 10 may be located in a hermetic cavitydefined by an encapsulation structure (not shown), as described inparticular in the publication by Dumont et al. entitled Current progresson pixel level packaging for uncooled IRFPA, SPIE Proceedings Vol. 8₃₅₃(2012). The encapsulation structure may be formed by a stack of variousthin layers, such as an encapsulation layer for example made ofamorphous silicon deposited by CVD or iPVD, covered with a sealing andanti-reflective layer, for example made from various sub-layers ofgermanium and zinc sulfide, deposited for example by EBPVD, IBS or thelike. Such an encapsulation structure is described notably in patentapplication EP_(3067675.)

The sensitive material 15 is based on vanadium oxide VO_(x), that is tosay that it is formed of what is called a base compound made from avanadium oxide VO_(x) to which at least nitrogen N as additionalchemical element, and possibly, in addition to nitrogen, boron B and/orcarbon C has been added. An additional chemical element is a chemicalelement intentionally added to the base compound, that is to say thevanadium oxide. The sensitive material 15 is amorphous, that is to saythat it contains substantially no crystalline phases. Moreover, it hasan electrical resistivity of between 1 Ω·cm and 30 Ω·cm, whichcorresponds to an amount of oxygen x, defined as the ratio between thenumber of oxygen atoms and the number of vanadium atoms, of between 1.42and 1.94 to within plus or minus 0.05. In addition, it has a homogeneouschemical composition, that is to say that its chemical composition asdefined in an elementary volume of the order of 3 nm in diameter isinvariant on a large scale (in at least 90%, 95% or even 99% of itsvolume).

The base compound is amorphous and is based on VO_(x)X, with x beingbetween 1.42 and 1.94, to within plus or minus 0.05, and preferablybeing between 1.56 and 1.94, to within 0.05. It does not show astoichiometric form. It is thus distinguished from stoichiometriccompounds such as VO₂, V₂O₅, V₃O₅. As stated above, the compound withthe empirical chemical formula V₂O₅ in this case has 5 oxygen atoms per2 vanadium atoms (x=5/2), and the compound V₃O₅ has 5 oxygen atoms per 3vanadium atoms (x=5/3). It will be noted here that the stoichiometriccompound V₃O₅ is a compound that cannot be obtained under the usualproduction conditions for such a microbolometer VO_(x) base compound(temperature usually less than the maximum thermal budget of the readoutcircuit located in the substrate 2, that is to say less than 400° C.).Therefore, the VO_(x) sensitive material according to the invention mayhave an amount x equal to 1.67 without however corresponding to thestoichiometric form V₃O₅. Moreover, with regard to the V₂O₃stoichiometric compound, there is virtually zero probability that such abase compound, that is to say that is amorphous and having an electricalresistivity of between approximately 1 Ω·cm and 30 Ω·cm, will be able toform a single V₂O₃ stoichiometric crystalline phase after annealing atthe temperature T_(r). Therefore, even for an amorphous base compoundhaving an amount of oxygen of about 1.5, therefore to within 0.05,several stoichiometric crystalline phases that differ in terms of theiramount of oxygen are therefore likely to be formed after annealing atT_(r), including the V₂O₃ crystalline phase. Whatever the case, if theamount of oxygen of the amorphous base compound is between 1.56 and1.94, to within 0.05, a single V₂O₃ stoichiometric crystalline phasecannot form after annealing at T_(r). It will moreover be noted that, ifthe base compound or the sensitive material has an amount of oxygen x ofbetween 1.56 and 1.94, to within 0.05, the native electrical resistivityis then between approximately 2 Ω·cm and 30 Ω·cm.

The sensitive material 15 then corresponds to a modified compound, thatis to say that it corresponds to the base compound that has beenmodified by the addition of at least one additional chemical element,here nitrogen N and possibly, in addition to nitrogen, boron B and/orcarbon C.

The amount of additional chemical element, specifically the number ofnitrogen atoms and possibly, in addition to nitrogen, of boron and/orcarbon atoms to that of vanadium, is chosen so as to give the sensitivematerial, which has been exposed to the temperature T_(r) for theduration Δt_(r), an electrical resistivity ρ_(a|r) at ambienttemperature at least equal to 50% of its native value ρ_(a). At leastequal is understood to mean greater or equal. The native value pa of theelectrical resistivity is that of the sensitive material before it hasbeen exposed to the temperature T_(r) for Δt_(r).

The values of the temperature T_(r) and of the duration Δt_(r) are suchthat the first compound based on VO_(x) (without the additional chemicalelement) has an electrical resistivity at ambient temperature less than50% of its native value. These are values of the temperature and theduration of the thermal exposure to which the sensitive material 15 willbe subjected in the subsequent steps of manufacturing themicrobolometer.

The amount of nitrogen and possibly of boron and/or carbon is thengreater than or equal to what is called an effective value, or effectiveamount. The effective amount is the minimum non-zero amount of nitrogenand possibly of boron and/or carbon starting from which the sensitivematerial, having undergone a step of exposure to the temperature T_(r)for the duration Δt_(r), has an electrical resistivity ρ_(a|r) atambient temperature at least equal to 50% of the native value pa of saidsensitive material at ambient temperature. The ambient temperature maybe equal to 30° C. The temperature T_(r) is greater than the ambienttemperature, and is preferably greater than or equal to 280° C., andpreferably greater than or equal to 300° C. It may be less than or equalto 400° C. The duration Δt_(r) is preferably greater than or equal to afew minutes or tens of minutes, or even a few hours.

In other words, when the sensitive material, the amount of nitrogen andpossibly of boron and/or carbon in which is greater than or equal to theeffective amount, has not been exposed to the temperature T_(r) for theduration Δt_(r), its electrical resistivity at ambient temperature hasthe native value ρ_(a). After thermal exposure to T_(r) for Δt_(r), thesensitive material then has an electrical resistivity ρ_(a|r) at ambienttemperature at least equal to 50% of the native value ρ_(a).

The effective amount depends notably on the base compound underconsideration, as well as on the chosen values of the temperature T_(r)and of the duration Δt_(r) of the thermal exposure. A person skilled inthe art is able to determine the effective amount, that is to say theminimum amount of nitrogen and possibly of boron and/or carbon, to beadded to the base compound so that the sensitive material has anelectrical resistance ρ_(a|r) at ambient temperature at least equal to50% of the native value ρ_(a). The effective amount may be chosen, ifnecessary, such that the value ρ_(a|), is greater than 50%, for exampleat least equal to 75%, or even at least equal to 90% of the native valueρ_(a).

The electrical resistivity at ambient temperature of the sensitivematerial may be determined using a conventional four-point measurementtechnique, and the atomic composition of the sensitive material, andtherefore the amount of the additional chemical element, may notably bedetermined by NRA (Nuclear Reaction Analysis), by RBS (RutherfordBackscattering Spectroscopy), by SIMS (Secondary Ion Mass Spectrometry),by XPS (X-ray Photoelectron Spectroscopy), using suitable standards.

It is known that the electrical resistivity at ambient temperature of asensitive material consisting of vanadium oxide VO_(x) may drop in valuewhen it has been exposed to a temperature of 300° C. or 400° C., inparticular under an inert atmosphere (under nitrogen), as described inthe publication by Venkatasubramanian et al. entitled Correlation oftemperature response and structure of annealed VO _(x) thin films for IRdetector applications, J. Vac. Sci. Technol. A 27(4), 2009, 956-961.Thus, a sensitive material consisting of vanadium oxide, and thereforewithout an additional chemical element such as nitrogen, boron andcarbon, has an electrical resistivity ρ_(a|r) at ambient temperature ofthe same order of magnitude as its native value ρ_(a) after exposure toa temperature of the order of 200° C. under an inert atmosphere.However, the electrical resistivity ρ_(a|r) drops by an order ofmagnitude, or even several orders of magnitude, when the sensitivematerial has been exposed to a temperature of 300° C. or 400° C. for 10min or 30 min under an inert atmosphere.

However, the inventors have found that the addition of a sufficientamount of nitrogen to a sensitive material based on vanadium oxide, andpossibly, in addition to nitrogen, of boron and/or carbon, surprisinglymakes it possible to improve the thermal stability of the sensitivematerial during thermal exposure to high temperatures, for example ofthe order of 300° C. or even more, for several tens of minutes, and moreprecisely to limit or even eliminate possible 1/f noise degradation ofthe sensitive material following the thermal exposure step.

The sensitive material based on vanadium oxide, with the addition of asufficient amount of nitrogen, and possibly of boron and/or carbon, thenhas an electrical resistivity ρ_(a|r) at ambient temperature at leastequal to 50% of the native value ρ_(a). The sufficient amount is anamount greater than or equal to the determined effective amount. Such asensitive material then does not exhibit significant degradation of itselectrical properties, and notably of its electrical resistivity atambient temperature and of its noise, following the microbolometermanufacturing process which includes at least one step of exposing thesensitive material to T_(r) for Δt_(r), and steps of depositing thinlayers, of encapsulating the microbolometer in a hermetic cavity, oreven of activating a getter material.

More precisely, it appears that exposing a compound based on VO_(x),when it is amorphous and has a native electrical resistivity ρ_(a) atambient temperature of between 1 ∩·cm and 30 ∩·cm, to a temperatureT_(r) for a duration Δt_(r) such that its electrical resistivity ρ_(a|r)is at least less than 50% of its native value ρ_(a), also causes adegradation of 1/f noise, without the TCR coefficient otherwise beingaffected. It is recalled that 1/f noise, also called flicker noise orlow-frequency noise, stems in particular from fluctuations in themobility and/or density of free carriers.

Moreover, an amorphous compound based on a vanadium oxide VO_(x), forwhich the electrical resistivity is between 1 Ω·cm and 30 Ω·cm, is notlikely to form a single stoichiometric crystalline phase, afterannealing at T_(r), under the usual conditions for producing such aVO_(x) base compound of a microbolometer (temperature less than 400°C.). In such an electrical resistivity range, the base compound has anamount of oxygen x of the order of approximately 1.42 to 1.94.Approximately is understood to mean here that the absolute uncertaintyis ±0.05. As indicated above, the electrical resistivity of the basecompound may be between 2 Ω·cm and 30 Ω·cm, its amount of oxygen x thenbeing between 1.56 and 1.94, to within 0.05.

The inventors have found that the evolution of the 1/f noise of such abase compound as a function of the thermal exposure temperature T_(r)for a given duration Δt_(r) is correlated with that of the electricalresistivity, but is not correlated with that of the TCR coefficient.

FIG. 2A thus illustrates an example of the evolution of the electricalresistivity ρ_(a|r) of a base compound made of VO_(1.8) (not containingnitrogen) as a function of the temperature T_(r) for a duration Δt_(r)equal to go min. The electrical resistivity ρ_(a|r) thus remainsconstant and equal to approximately 10 Ω·.cm up to the temperature T_(r)of approximately 280° C. It then exhibits a strong decrease, inparticular between 300° C. and 325° C.

It moreover appears that the evolution of the value of the temperaturecoefficient of resistance (TCR) as a function of the temperature T_(r)does not seem to be correlated with that of the electrical resistivitywhen the degradation of the electrical resistivity is linked to thethermal exposure to T_(r) for Δt_(r).

FIG. 2B thus illustrates the values of various measurements of the TCRcoefficient (in arbitrary units) of a base compound made of VO_(x), theelectrical resistivity of which is between 5 Ω·cm and 15 Ω·cm, as afunction of the electrical resistivity ρ_(a|r) after annealing at 310°C. for a duration Δt_(r) of 90 min (hollow circles). Values of the TCRcoefficient for this same type of VO_(x) base compound without annealingat the temperature T_(r) are also indicated (solid diamonds). It appearsthat the TCR coefficient of such a VO_(x) base compound remainssubstantially constant, this being the case whether or not the basecompound has been subjected to thermal exposure at a high temperatureT_(r) of 310° C.

On the other hand, it appears that the i/f noise associated with thistype of VO_(x) base compound exhibits an increase that is correlatedwith the reduction in the electrical resistivity ρ_(a|r) when this isdue to the thermal exposure to the temperature T_(r) for the durationΔt_(r).

FIG. 2C thus illustrates the values of various measurements of aparameter N_(1/f) representative of the i/f noise of the VO_(x) basecompounds of FIG. 2B as a function of the electrical resistivity ρ_(a|r)after annealing at 310° C. for a duration Δt, of 90 min (soliddiamonds). Values of this 1/f noise parameter for these VO_(x) basecompounds without annealing at the temperature T_(r) are also indicated(solid diamonds). While the 1/f noise remains substantially constantregardless of the value of the electrical resistivity for these

VO_(x) base compounds without annealing at T_(r), it appears thatannealing the VO_(x) base compounds at a temperature of 310° C. for 90min causes a significant increase in 1/f noise.

The parameter N_(1/f) representative of the 1/f noise is estimated herefrom the spectral analysis of a reference electric current flowing inthe sensitive material. To this end, the sensitive material is biasedwith a direct (DC) voltage source set so as to dissipate the referencecurrent into the sensitive material. A very low noise voltage source isalso used so as not to bias the measurement of the noise of thesensitive material. The reference current is thus marred by the onlynoise current of the sensitive material. This current is then amplifiedby a transimpedance amplifier that delivers a voltage output signal thatis the image of the input current. The voltage signal is sampled,digitized and processed digitally (Fourier transform) in order to obtainits spectrum. The amplitude of the i/f noise may be obtained by readinga particular point of the spectrum, for example at 1 Hz, or using aleast squares calculation method on the low-frequency part of thespectrum where the manifestation of the i/f noise is the mostpronounced.

It therefore appears that exposing an amorphous VO_(x) base compoundhaving a native electrical resistivity ρ_(a) at ambient temperature ofbetween 1 Ω·cm and 30 Ω·cm (that is to say non-stoichiometric form) to atemperature T_(r) for a duration Δt_(r) such that its electricalresistivity ρ_(a|r) drops with respect to its native value ρ_(pa) alsocauses a degradation of the 1/f noise, without the TCR coefficientotherwise being affected.

This increase in the 1/f noise of such a VO_(x) base compound may be theresult of the onset of crystallization of the compound, in whichdistinct crystalline phases appear, which differ from one another interms of the amount of oxygen, these crystalline phases then beingstoichiometric forms. Thus, by way of example, the at least partialcrystallization of an initially amorphous VO_(x) base compound, where xis of the order of i.8, results in the appearance of variousstoichiometric crystalline phases, including VO₂ and V₂O₅ (that is tosay x=2.5). The increase in 1/f noise could thus be linked to theappearance of several stoichiometric crystalline phases, differing fromone another in terms of the amount of oxygen, and therefore in terms ofthe loss of the homogeneous character of the chemical composition of thesensitive material, and therefore of its local electrical properties.

In this respect, FIGS. 3A and 3B illustrate examples of Raman spectra ofan initially amorphous VO_(x) base compound (therefore without nitrogenadded), where x is equal to approximately 1.85, for various exposures toa temperature T_(r) for a duration Δt_(r) of 90 min. The Raman spectraof FIG. 3A are centered on a Raman shift range ranging fromapproximately 100 to 300 cm⁻¹, and those of FIG. 3B are centered on aRaman shift range ranging from approximately 700 to 950 cm⁻¹. The curveA_(o) corresponds to the Raman spectrum of a VO_(1.85) compound withoutannealing, and the curve A_(o) corresponds to that of the support onwhich the compound in question rests. The curves A₁, A₂, A₃ and A₄correspond to the Raman spectrum of the VO_(1.85) compound that hasundergone exposure for 90 min to temperatures T_(r) of 300° C., 310° C.,320° C. and 330° C., respectively. It appears that the peak at 149 cm⁻¹appears and increases in terms of intensity when the temperature T_(r)increases, this peak being associated with the stoichiometriccrystalline phase V₂O₅ (x=2.5). Similarly, the peaks at 197 cm⁻¹ and 224cm⁻¹ associated with the stoichiometric crystalline phase VO₂ appear andincrease in terms of intensity with the temperature T_(r).Correlatively, the peak at 860 cm⁻¹ associated with the amorphouscharacter of the base compound decreases as the temperature T_(r)increases.

It thus appears that the high-temperature exposure of an initiallyamorphous base compound made from VO_(x) and not containing anynitrogen, and whose native electrical resistivity is between 1 Ω·cm and30 Ω·cm, leads to at least partial crystallization of the sensitivematerial, which results in a drop in its electrical resistivity and inan increase in 1/f noise. In other words, electrical resistivity atambient temperature is a parameter representative of the amorphous ornon-amorphous character of the compound, as well as of 1/f noise.Therefore, by adding a sufficient amount of nitrogen to the basecompound as an additional chemical element in order to obtain a modifiedcompound, it is possible to limit the crystallization or even to pushback the crystallization threshold of the modified compound, andtherefore to limit or even eliminate 1/f noise degradation.

As mentioned above, the manufacturing process then comprises a step ofadding nitrogen as an additional chemical element to the base compound,so as to obtain a modified compound. This has a native electricalresistivity substantially equal to that of the base compound. This isbetween 1 Ω·cm and 30 Ω·cm, which corresponds to a non-stoichiometricamount of oxygen x. The amount of nitrogen to be added to the basecompound is thus determined such that the compound thus modified, whenexposed to the temperature T_(r) for the duration Δt_(r) determinedbeforehand, has an electrical resistivity ρ_(a|r) greater than or equalto its native value. The partial crystallization of the modifiedcompound is thus limited, giving rise to stoichiometric crystallinephases that differ from one another in terms of the amount of oxygen x,and 1/f noise degradation is also limited. The thermal stability of theproperties of the sensitive material is thus improved when it issubsequently exposed to the temperature T_(r) for the duration Δt_(r).

Such a sensitive material is then particularly advantageous in thecontext of a collective manufacturing process for manufacturing an arrayof microbolometers of an electromagnetic radiation detection device.Specifically, in a thermal exposure step, the temperature field mayexhibit spatial inhomogeneities within a thin-film deposition reactor oran annealing furnace, which may result in a dispersion of the electricalproperties of the microbolometers. Thus, by using the sensitive materialwith a sufficient amount of nitrogen and possibly of boron and/orcarbon, the microbolometers exhibit better thermal stability duringthermal exposure to the temperature T_(r) thus reducing the dispersionof the electrical properties of the microbolometers.

Moreover, the sensitive material may also comprise a transition metalbelonging to period 4 of the periodic table of the elements, that is tosay scandium Sc, titanium Ti, chromium Cr, manganese Mn, iron Fe, cobaltCo, nickel Ni, copper Cu and/or zinc Zn. It may also comprise otherchemical elements, for example yttrium Y, niobium Nb, molybdenum Mo,tantalum Ta, tungsten W, inter alia.

FIG. 4 illustrates an example of the evolution of the electricalresistivity ρ_(a|r) at ambient temperature of the sensitive material asa function of the thermal exposure temperature T_(r), for a sufficientamount of added nitrogen. The exposure duration Δt_(r) is equal to 90min. This example thus demonstrates the increase in the thermalstability range that the sensitive material exhibits when it contains asufficient amount of nitrogen, that is to say an amount greater than orequal to the effective amount.

Here, the sample of the VO_(x) sensitive material is obtained using ionbeam sputtering (IBS), for example by sputtering a vanadium target underan oxidizing atmosphere at a partial oxygen pressure for example of theorder of approximately 10⁻⁴ Torr. The sample of the VO_(x)N_(w)sensitive material was obtained by IBS sputtering in an atmospherecomprising nitrogen in addition to oxygen. The amount of oxygen x isequal here to approximately 1.75

The curve C1 corresponds to the evolution of the electrical resistivityρ_(a|r) of a compound based on VO_(x), where x=approximately 1.75, notcontaining nitrogen. It has a native electrical resistivity of the orderof approximately 3 to 4 Ω·m. The electrical resistivity ρ_(a|r) becomesless than 50% of its native value starting from the temperature ofapproximately 310° C. for an exposure of 90 min.

The curve C2 illustrates the evolution of the electrical resistivityρ_(a|r) as a function of the temperature T_(r) for a VO_(1.75)N_(w),sensitive material, where the amount w of nitrogen corresponding to 2.8%at, that is to say the ratio of the number of nitrogen atoms to that ofvanadium, is in this case equal to 0.079. It appears that the thresholdvalue T_(th) of the sensitive material, at which the electricalresistivity ρ_(a|r) is at least equal to 50% of the native value ρ_(a),also increases as the amount w of nitrogen in the sensitive materialincreases. It thus changes from 310° C. for VO_(1.75) without theaddition of nitrogen to approximately 330° C. for VO_(1.75)N_(0.079).

The curve C3 illustrates the evolution of the electrical resistivityρ_(a|r) as a function of the temperature T_(r) for a VO_(1.75)N_(w)sensitive material, where the amount w of nitrogen corresponding to 3.8%at, that is to say the ratio of the number of nitrogen atoms to that ofvanadium, is in this case equal to 0.109.

The effective amount of nitrogen such that the sensitive material has atleast 50% of its native value after having been exposed to 310° C. for90 min is thus here 0.079 for x being equal to approximately 1.75.Therefore, in this example, a sensitive material based on vanadium oxideto comprising an amount w of nitrogen equal to at least 0.079 exhibitssignificant thermal stability allowing it to be exposed to temperaturesof up to 330° C. for at most 90 min, while at the same time having itselectrical properties preserved, in particular having a limited 1/fnoise degradation. For a sensitive material having an amount of oxygenbetween 1.42 and 1.94, the effective amount of nitrogen here is at leastequal to 0.07, for an exposure to approximately 310° C. (to within 5°C.) for 90 min.

The value of the effective amount of nitrogen depends on the exposuretemperature T_(r) and on the exposure duration Δt_(r). A person skilledin the art is then able to determine the value of the effective amountof nitrogen to be added to the compound based on VO_(x) on the basis ofthe thermal characteristics T_(r) and Δt_(r) of the exposure step.

The sensitive material may comprise, in addition to nitrogen, an amountof boron and/or carbon. The amount of additional chemical elements, thatis to say the amount of nitrogen and that of boron and/or carbon, isdetermined such that the sensitive material 15 has an electricalresistivity ρ_(a/r) at ambient temperature at least equal to 50% of thenative value ρ_(a).

A VO_(x)WB_(y) sensitive material containing, in addition to nitrogen,an amount of boron may be produced by implanting boron in a VO_(x)N_(w)material produced beforehand by IBS sputtering in an oxidizingatmosphere containing nitrogen. The same applies in the case of addingcarbon to the VO_(x)N_(w) compound. Other techniques for producing theVO_(x)N_(w) sensitive material with the addition of boron and/or carbonmay be used.

It appears that the sensitive material based on VO_(x)N_(w) with asufficient amount of nitrogen and possibly of boron and/or carbonexhibits a relative variation in the electrical resistivity p_(a|r) as afunction of the temperature T_(r) that is particularly low when thethermal exposure temperature is less than or equal to its thresholdvalue T_(th). This then makes it possible to limit the dispersion ofelectrical properties of the microbolometers resulting from any spatialinhomogeneities in the temperature field within the deposition reactoror the annealing furnace.

Thus, one aspect of the invention relates to a process for manufacturingat least one microbolometer 10, and advantageously, to a collectivemanufacturing process for manufacturing an array of bolometers 10 of adetection device 1.

The manufacturing process comprises at least one step in which thesensitive material 15 of the microbolometers 10 is subjected to atemperature T_(r) for the duration Δt_(r).

The values of the temperature T_(r) and of the duration Δt_(r) aredetermined such that the base compound of the sensitive material 15(therefore without the additional chemical element) has an electricalresistivity ρ_(a|r) at least less than 50% of its native value ρ_(a).These are in particular the temperature and the duration to which thesensitive material will subsequently be subjected in the subsequentsteps of manufacturing the microbolometer.

The manufacturing process then comprises a preliminary step ofdetermining the effective value of the amount of at least one additionalchemical element, specifically nitrogen and possibly, in addition tonitrogen, of boron and/or carbon, starting from which the sensitivematerial 15, having undergone exposure to the temperature T_(r) for theduration Δt_(r), has an electrical resistivity P_(a|r) at ambienttemperature at least equal to 50% of its native value ρ_(a).

In a step of producing the absorbent membrane, the sensitive material 15is deposited in a thin layer on the support layer 20, which rests on asacrificial layer. It comprises a sufficient amount of nitrogen, andpossibly of boron and/or carbon, that is to say an amount greater thanor equal to the determined effective value. It may have a thickness ofthe order of a few ten to a few hundred nanometers, for example between10 nm and 500 nm, for example equal to 80 nm.

The sensitive material 15 may be obtained by IBS sputtering under anatmosphere containing nitrogen, and possibly by ion implantation ofboron and/or carbon. The temperature may be the ambient temperature. Theatmosphere is oxidizing when the thin layer of VO_(x) is produced andthe partial oxygen pressure may be of the order of approximately 10⁻⁵ to10⁻⁴ Torr, for example equal to 6.10 ⁻⁵ Torr, depending on the desirednative value of the electrical resistivity of the sensitive material.The value of the partial oxygen pressure and the content of nitrogen inthe atmosphere, or else the content of boron and/or carbon, may beobtained from calibration curves obtained beforehand. Other productiontechniques may be used, such as atomic layer deposition (ALD), or thedeposition of the sensitive material through reactive cathodicsputtering with a metal or vanadium oxide target, possibly followed byimplantation of boron and/or carbon.

It is then possible to cover the VO_(x)N_(w) compound with theprotective layer of silicon nitride, with a thickness for example of 10nm deposited by PECVD. Boron and/or carbon may then possibly be added byperforming several successive implantations. Thus, for a thin layer ofthe VO_(x)N_(w) compound with a thickness of 80 nm covered with the SiNprotective layer with a thickness of 10 nm, three successiveimplantations of boron may be carried out, starting from doses of theorder of 10¹⁶ at/cm² and for implantation energies of the order ofaround ten to a few tens of KeV. Other techniques for producing thesensitive material may be used, or even other variants of theabovementioned deposition techniques. Thus, for the addition of carbon,it is possible to sputter a target containing only vanadium, under anoxidizing atmosphere containing nitrogen and CO or CO₂.

The manufacturing process then comprises at least one step in which thesensitive material is exposed to a temperature greater than the ambienttemperature, and less than or equal to T_(r) for a duration less than orequal to Δt_(r). The exposure temperature may be equal to T_(r) and bebetween 300° C. and 400° C., and the duration may be equal to Δt_(r) andbe of the order of a few minutes to a few hours. This thermal exposurestep may be carried out under an inert atmosphere or under vacuum.

This may thus involve depositing the protective thin layer 22 of siliconnitride through PECVD, depositing at least one of the thin layersforming the encapsulation structure that defines the hermetic cavity,eliminating the sacrificial layer or sacrificial layers used to producethe suspended membrane or the encapsulation structure, or evenactivating a getter material arranged in said hermetic cavity.

Particular embodiments have just been described. Various modificationsand variants will be apparent to a person skilled in the art.

1. A process for manufacturing at least one microbolometer comprising a sensitive material for at least limiting noise degradation of said sensitive material, said sensitive material being formed of a first compound based on vanadium oxide and at least nitrogen as additional chemical element, the process comprising the following steps: a step of producing the sensitive material in a thin layer; a step of exposing the sensitive material to a temperature T_(r) greater than the ambient temperature, for a duration Δt_(r), this thermal exposure step being performed after the step of producing the sensitive material, the temperature T_(r) and the duration Δt_(r) being such that said first compound, being amorphous and having a native electrical resistivity value at ambient temperature of between 1 Ω·cm and 30 Ω·cm, having undergone a step of exposure to the temperature T_(r) for the duration Δt_(r), has an electrical resistivity at ambient temperature less than 50% of its native value; determining a non-zero what is called effective amount of the additional chemical element added to said first compound, thus forming a modified compound, starting from which the modified compound, having undergone a step of exposure to the temperature T_(r) for the duration Δt_(r), has an electrical resistivity ρ_(a|r) at ambient temperature greater than or equal to 50% of its native value ρ_(a); in said step of producing the sensitive material in a thin layer, forming the thin layer of said modified compound having an amount of the additional chemical element greater than or equal to the effective amount determined beforehand, the sensitive material being amorphous, having a native electrical resistivity value ρ_(a) at ambient temperature of between 1 Ω·cm and 30 Ω·cm, and a homogeneous chemical composition; and following said step of exposing the sensitive material to the temperature T_(r) for the duration Δt_(r), said sensitive material then has a noise whose degradation has been at least limited.
 2. The manufacturing process as claimed in claim 1, wherein the step of exposing the sensitive material comprises a step of depositing a protective layer covering the sensitive material.
 3. The manufacturing process as claimed in claim 1, wherein the step of exposing the sensitive material comprises a step of depositing an encapsulation layer transparent to the electromagnetic radiation to be detected and intended to define a cavity in which the microbolometer is located.
 4. The manufacturing process as claimed in claim 1, wherein the temperature T_(r) is greater than or equal to 280° C.
 5. The manufacturing process as claimed in claim 1, wherein the duration Δt_(r) is greater than or equal to 90 min.
 6. The manufacturing process as claimed in claim 1, wherein the sensitive material is produced at a temperature less than the temperature T_(r).
 7. A microbolometer comprising a sensitive material made of a first compound based on vanadium oxide and at least nitrogen as additional chemical element, characterized in that wherein the sensitive material: is amorphous, has an electrical resistivity at ambient temperature of between 1 Ω·cm and 30 Ω·cm, a homogeneous chemical composition, and an amount of nitrogen, defined as a ratio of the number of nitrogen atoms to that of vanadium, at least equal to 0.070.
 8. The microbolometer as claimed in claim 7, wherein the amount of oxygen, defined as a ratio of the number of oxygen atoms to that of vanadium, is between 1.42 and 1.94, to within plus or minus 0.05.
 9. The microbolometer as claimed in claim 7, wherein the sensitive material is covered by a protective layer of silicon nitride.
 10. A device for detecting electromagnetic radiation, comprising an array of microbolometers as claimed in claim 7, the microbolometers being arranged in at least one hermetic cavity delimited by an encapsulation structure transparent to electromagnetic radiation to be detected, the encapsulation structure comprising at least one layer made of amorphous silicon.
 11. The detection device as claimed in claim 10, comprising a getter material located in the hermetic cavity.
 12. The manufacturing process as claimed in claim 1, wherein the temperature T_(r) is equal to 310° C., to within 5° C. 